One gets a big head watching “Frank.” Not to mention a swollen heart. But does size really matter? You bet it does, especially when it concerns Michael Fassbender. And we’re not talking about his you know-what that he oh-so immodestly dangled in “Shame.” No, this time we’re talking about a talent so enormous it’s caused his noggin to swell 10 times its size – more than enough to cause “Lucy” cranium envy. But it’s all fake … the head, that is – a mass of bulbous fiberglass painted to look a little like Davey from the TV puppet show “Davey and Goliath.” And behind it all is Fassbender, pulling the strings, face unseen, sans any ugly wires.

So, what’s the point? There isn’t one, really; there are many. From mental illness to artistic integrity to our selfish need to force others to conform to the way we project them to be. And it’s all tightly, and neatly, packaged into a 90-minute musical mind-blower, courtesy of director Lenny Abrahamson. He’s the brains, but the heart of these head games belongs to writers Peter Straughan and Jon Ronson, the “Men Who Stare at Goats” team seeking redemption for that George Clooney flop with a film so funny and original forgiveness is unquestioned.

They say you should write what you know, and what Ronson knows is what it’s like to catch the rock ’n’ roll bug and allow it to infect your life. It bit him in his youth, when he was a wannabe keyboardist and songwriter. It advanced unchecked after hooking up with Brit singing legend Frank Sidebottom and his band. Frank, aka the late comedian Chris Sievey, became infamous for the large fake head he wore to hide his insecurities. And like Ronson, played here by sensational newcomer Domhnall Gleeson (son of Brendan), the new kid in the band becomes obsessed with the mind of the man behind the head. This is his story, at least in theory. But like Ronson, you become more intrigued by Frank and his various neuroses, most notably his fear of success. Or, worse, selling out.

It’s a familiar scenario, but what makes this one unique is the level of authenticity Ronson brings to the plate, as he puts you smack in the shoes of his alter-ego, Jon, a dreamer living his dream – even though that dream may be beyond his level of talent and comprehension. As Jon, our conduit into this fictionalized version of Sidebottom’s life, Gleeson is a natural at projecting the wide-eyed wonder and terror of a nobody who suddenly has the chance to be a somebody. He’s even better at jonesing on the prospect of fame, a goal not shared by his demonstrative bandmates Clara (a hilariously caustic Maggie Gyllenhaal), Nana (real-life drummer Carla Azar), lead guitarist Baraque (Francois Civil), and of course, Frank.

Page 2 of 2 - His best scenes, though, are the telling, late-night conversations Jon has with his newfound idol. You sense Jon’s urgency and desperation as he continually attempts, but fails, to penetrate Frank’s intimidating mask, a get-up he even sleeps and showers in. Jon thinks he knows Frank, but all he knows is what he fantasizes his hero to be. And living up to that worship slowly begins to take a toll on Frank and his mental stability. It’s only a matter of time before his head literally cracks.

That’s when Fassbender really goes to work. Amazingly, he does it all from behind a beachball-sized plastic head. Facial expressions? Who needs them when Fassbender can so richly communicate Frank’s joys and anxieties merely through posture and voice inflections, which are topped only by his singing, especially on the heart-tugging “I Love You All.” If “Frank” were a bigger movie, Fassbender would be a shoo-in for an Oscar nod. Ditto for Gyllenhaal, who is a hoot as Jon’s acerbic nemesis who can clearly see Jon for the sycophant he is.

And let’s not forget Fassbender’s “12 Years a Slave” co-star, Scoot McNairy, rampantly stealing scenes as the unfazed manager of Frank’s incomparable band with the unpronounceable name, Soronprfbs. It is he who entices Jon to step “through the looking glass,” an invitation that takes the naive lad on a whirlwind journey from his boring, working-class Brighton neighborhood to the cusp of stardom at the South-by-Southwest Festival in Texas.

Who will win the ongoing knockdown-dragout to control Frank? Jon or Clara? It’s all quite dramatic, but served up with so much droll humor and quasi-slapstick that it never weighs heavy. It’s quirky, yes. Maybe a little too quirky at points. But it also practices what it preaches about the purity and honesty of art. It’s not about the money; it’s about the soul, and “Frank” definitely has that. It also has Fassbender, whose praises can’t be sung enough. He’s music to the movies.